1. Field of the Invention
The present invention relates to catalytic compositions consisting of zeolite Beta (as such or modified) and a ligand which can be used particularly in processes for the alkylation of aromatic hydrocarbons with olefins, in particular benzene with light olefins and more specifically with ethylene to give ethylbenzene and with propylene to give cumene. The catalytic composition of the present invention is also particularly suitable in the transalkylation of aromatic hydrocarbons with polyalkylated aromatic hydrocarbons, especially of benzene with diethylbenzene to give ethylbenzene and benzene with diisopropylbenzene to give cumene.
2. Description of the Background
Former alkylation processes, still widely used in the petrolchemical industry for the production of the two organic intermediates quoted above, involve the use of a catalyst based on phosphoric acid and infusorial earth in a fixed bed for cumene and AlCl.sub.3 in slurry for ethylbenzene. The possibility of substituting these catalysts with non-polluting, non-corrosive and regenerable materials such a zeolitic catalysts has been known for some time.
There are mainly two types of problems however arising from the use of zeolitic catalysts in alkylation reactions such as those listed above:
a higher percentage of polyalkylated by-products; PA0 a more rapid deactivation of the zeolitic catalyst. PA0 by non-thermal regeneration techniques in situ as to allow for minimum shiftings or which may easily be accomplished with respect to normal running conditions in the reaction; PA0 intervening on the catalyst.
The first problem compels the use of a second reactor, if the alkylation step is carried out at an insufficiently high temperature, for recovering said by-products, mainly consisting of dialkylates, by transalkylation with benzene, or their direct recycling into alkylation if instead this step is carried out at a sufficiently high temperature.
On the other hand, the second problem, a more rapid deactivation of the catalyst, compels a certain frequency of necessary thermal regenerations which will be greater in number the shorter the duration of the single reaction cycle intended as the duration of the catalyst between two successive thermal regenerations. It is in fact evident that a greater duration of the single reaction cycle will lead to a lower total number of thermal regenerations, with the same complete duration of the catalyst, and on the other hand this complete duration may in turn depend on the total number of thermal regenerations undergone by the catalyst itself and can therefore increase with a greater duration of the single reaction cycle.
The increase in duration per single reaction cycle and consequently in the productivity can be basically obtained by proceeding in two directions:
Various patents claim processes and expedients in the first direction indicated; for example patent PCT/92/02877 describes a process for extending the duration of the single reaction cycle between two thermal regenerations for catalysts based on zeolites in alkylation reactions; this process basically consists in the continuous feeding of a moderate concentration of H.sub.2 O together with the reagents.
U.S. Pat. No. 5,518,897 discloses instead a process for reactivating catalysts based on zeolites in alkylation reactions by interruption of the olefin stream and substitution with a moderate stream of hydrogen under certain conditions and for a certain period of time.
This would enable the catalytic activity to be brought back to normal values and thus lengthen the duration of the single reaction cycle before thermal regeneration. As far as the second point is concerned, i.e. the preparation of a catalyst with particular duration characteristics per single reaction cycle, it is possible to cite for example U.S. Pat. No. 4,870,222 which claims an alkylation and transalkylation process for producing cumene with the use of an amorphous silica/alumina catalyst in alkylation and a second catalyst based on mordant in transalkylation.
The catalyst based on mordant bound with alumina used in transalkylation is subjected to a modification treatment of the porous structure in order to obtain a higher Specific Surface Area value (SSA) equal to at least 580 m.sup.2 /g.
It is evident that the value is typical of the components zeolite mordant and alumina, used in the preparation of the catalyst and also obviously depends on the relative percentage actually present; the patent cites an example relating to a material containing 10% of ligand and which after the treatment claimed increases the SSA from 540 m.sup.2 /g to 620 m.sup.2 /g. This treatment creates a greater activity of the catalyst in transalkylation and also a longer duration as shown by the life tests described in the examples of the patent.